The present invention relates generally to capacitively-coupled touch sensors, and more particularly to capacitively-coupled “slider” sensors which respond to touch by generating a linear output signal relative to the position of a user's finger as it slides along the surface of a touch sensor. The invention relates still more particularly to such a linear capacitively-coupled touch sensor that is essentially independent of variations in finger pressure and various other “environmental” variables.
Capacitively-coupled touch sensors are well understood, and are becoming widely used in modern electronics to replace mechanical switches as input devices for command and control functions. In many common applications, an individual touch sensor performs a single function such as turning a lamp on or off. In previous designs, multiple individual capacitively-coupled touch sensors with digital outputs have been positioned side-by-side and utilized in conjunction with a microprocessor or the like that performs an interpolation process in order to compute a linear value indicating the present location of a user's finger touch. The interpolation technique typically requires use of at least 5 to 7 individual capacitively-coupled sensors to implement the foregoing “pseudo-linear” functionality.
Unfortunately, the use of multiple individual capacitively-coupled touch sensors and interpolation techniques as indicated above to provide pseudo-linear functionality may be too complex and costly for use in a modern laptop computer or other electronic device in which available space for switches and the like is at a premium. Furthermore, there are many other applications that include a row of individual capacitively-coupled touch sensors located side-by-side wherein it would be advantageous to be able to provide a linear output signal precisely indicating the user's finger touch location within the field of the individual capacitively-coupled touch sensors without the complexity and cost of performing such interpolation processes.
If a linear capacitively-coupled touch sensor which minimizes the number of leads required to integrate the function were available to be integrated in a die on which available area is at a premium, such a touch sensor could be advantageously utilized in various electronic devices. This would allow flexibility in the definition of switch functions in future designs of laptop computers and various other electronic devices. For example, the locations of discrete boundaries or “hot zones” of the individual inputs could be defined by silkscreen boundaries on external protective covering material. The linear positions of finger touch could determine various functions intended to be activated/deactivated under firmware control by touching various corresponding touch locations of a such a capacitively-coupled linear touch sensor.
FIG. 1 shows a plausible capacitively-coupled touch sensor system 10-1 which is believed to be novel. Capacitively-coupled sensor system 10-1 includes a capacitively-coupled touch sensor 4-1 which is sensitive to the linear position of a user's finger 19. When a signal source 7 applies a signal that is rich in harmonic content to a conductive center element or “plate” 2, the resulting signal is capacitively coupled from center plate 2 to outer plate 3 and generates a signal V6 that is developed across a load resistor RLOAD and is provided as an input to an amplifier 8. The amplifier may also perform some signal conditioning to remove unwanted noise or otherwise undesirable variations. A peak detector 12 detects a signal V9 generated on amplifier output 9 and generates a corresponding peak signal V13 of peak detector output 13. An ADC (analog-to-digital converter) 15 converts the analog signal V13 to a corresponding digital output signal DOUT on digital bus 17. A suitable processor (not shown) can process the digital information and compute a value along the x axis that represents the position at which finger 19 touches touch sensor 4-1.
In order to vary the capacitive coupling relative to finger touch position, finger 19 in effect becomes another capacitor plate of touch sensor 4-2. The effective area of the outer plate 3 involved in capacitive coupling with finger 19 increases as the finger touch position moves left to right along the x axis. Consequently, as the touch of finger 19 moves from left to right in FIG. 1, the effective capacitive coupling between outer plate 3 and inner plate 2 is proportional to the finger touch position along the x axis.
Unfortunately, the capacitive coupling is significantly affected by the above-mentioned variations in the touch pressure of Finger 19 (and also by variations in some other “environmental” parameters, such as humidity and user skin conductivity).
When finger 19 is positioned to the extreme left in the x direction along the longitudinal axis of inner conductive layer or plate 2, there is minimal capacitive coupling between outer conductive layer or plate 3 and inner conductive layer or plate 2. As finger 19 slides to the right, the capacitance or capacitive coupling between outer plate 3 (which includes continuous sections 3A, 3B, and 3C) and inner plate 2 increases because the amount of area of outer plate 3 being influenced by the presence of finger 19 increases. Consequently, the capacitive coupling between inner plate 2 and portions 3A and 3B of outer plate 3 increases as a function of finger position in the x direction. (To minimize unwanted variation of coupling when the finger 19 moves in the y direction, the geometry of inner conductive layer or plate 2 is minimized along with the separating gap.)
If the position of finger 19 moves slightly in the z (vertical) direction, and if the associated finger touch pressure is not maintained constant, then the amount of capacitive coupling between outer plate 3 and inner plate 2 increases if the finger pressure is increased and decreases if the finger pressure is decreased. This modifies the values of the signals V6, V9, V13, and DOUT in FIG. 1. The effect of variation of the finger pressure on the amount of the capacitive coupling between outer plate 3 and inner plate 2 ordinarily would be undesirable because such variation causes the determination of the present finger touch location to be inaccurate.
Therefore, it would be highly desirable to have a linear, capacitively-coupled touch sensor method and structure for eliminating the effect of finger pressure (and also various environmental variables) on the amplitude of the output signal VOUT produced in response to touch sensor 4-1. It also would be desirable to provide a capacitively-coupled touch sensor capable of functioning as a linear control switch or “slider” switch so as to provide an accurate linear output signal representative of the location of a user's finger touch without resorting to discrete sensors and mathematical interpolation.
Thus, there is an unmet need for a capacitively-coupled touch sensor and method capable of functioning as a linear control switch or “slider” switch in order to provide a linear output signal representative of the location of a user's finger touch.
There also is an unmet need for a capacitively-coupled touch sensor capable of functioning as a linear control switch or “slider” switch in order to provide an accurate linear output signal representative of the location of a user's finger touch without the complexity and cost of discrete sensors and performing mathematical interpolation of signals generated by those sensors.
There also is an unmet need for a capacitively-coupled touch sensor capable of functioning as a linear control switch or “slider” switch in which an output signal representative of the present location of the user's finger touch is independent of variations in finger touch pressure.
There also is an unmet need for a capacitively-coupled touch sensor capable of functioning as a linear control switch or “slider” switch in which an output signal representative of the present location of the user's finger touch is independent of variations in finger touch pressure and is also independent of variations in various environmental parameters.